MEMS Transducer Fabrication Using Sacrificial Layers
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Solution Overview
Problem
The challenge in fabricating MEMS transducers, such as capacitive microphones, lies in achieving accurate back-etching processes and integrating them with CMOS electronic devices, where tapering of back-etches alters electrode and membrane dimensions, affecting frequency response and sensitivity, and the singulation process damages fragile devices due to mechanical stress and residue issues.
Innovation Solution
The method involves depositing first and second sacrificial layers to protect the membrane and allow independent formation of the membrane size, followed by removing these layers to create movable MEMS transducers with defined cavities, and using bleed holes to improve air flow and frequency response, while also providing mechanical support during singulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a back-etch process is used to form the back-volume, then the device can be manufactured, but the sides of the back-volume converge inwards causing tapering that alters electrode and membrane dimensions and changes operating characteristics
Solution Approach 1:
The patent applies preliminary action by forming the membrane and electrode structures before performing the back-etch process. The membrane is deposited and patterned on the substrate, electrodes are formed, and then the back-etch is performed to create the back-volume. This sequence ensures that the membrane dimensions are defined by the deposition and patterning processes rather than being altered by the back-etch tapering effect.
Solution Approach 2:
The patent resolves the dimensionality issue by separating the membrane formation process (occurring on the front surface of the substrate) from the back-volume formation process (occurring on the back surface of the substrate). The membrane dimensions are controlled in the lateral dimension through photolithography patterning, while the back-etch operates in the vertical dimension to create the back-volume cavity, thus preventing tapering from affecting membrane dimensions.
2Reliability
If the membrane is made thin to achieve high sensitivity, then the device performance is improved, but the device becomes fragile and may be damaged during singulation
Solution Approach 1:
The patent applies beforehand cushioning by forming a support structure beneath the thin membrane during the fabrication process. A sacrificial layer is deposited and patterned to create a support structure that protects the thin membrane during subsequent processing steps, including the fragile singulation process. After singulation, the sacrificial layer is removed to leave the membrane free-standing and functional.
Solution Approach 2:
The patent segments the membrane support function from the membrane sensing function. The membrane itself remains thin and sensitive for detection, while a separate sacrificial support structure provides mechanical strength during fabrication and singulation. This segmentation allows the membrane to be thin for sensitivity while the support structure provides the necessary strength, and the support is temporarily present only when needed.
3Ease of manufacture
If standard CMOS processes are used for integration, then manufacturing compatibility is improved, but the back-etch process becomes difficult to perform accurately
Solution Approach 1:
The patent introduces an intermediary sacrificial layer that mediates between the CMOS-compatible fabrication process and the final device structure. The sacrificial layer is deposited using CMOS-compatible techniques, provides a well-defined template for the back-volume cavity, and can be selectively removed without affecting other device components. This intermediary enables precise back-volume formation while maintaining compatibility with standard CMOS manufacturing processes.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach ensures consistent membrane dimensions, improved frequency response, reduced mechanical stress during singulation, and enhanced operational characteristics by decoupling etching from membrane formation, leading to more reliable and functional MEMS devices.
Implementation Method 1
A back-volume 12 is formed using an etching process from below the substrate
Implementation Method 2
the flexible membrane 11 is free to move in response to pressure differences generated by sound waves
Implementation Method 3
capacitive transducers for detecting and/or generating pressure/sound waves
Data Source
AI summary
A MEMS device comprising a flexible membrane that is free to move in response to pressure differences generated by sound waves. A first electrode mechanically coupled to the flexible membrane, and together form a first capacitive plate. A second electrode mechanically coupled to a generally rigid structural layer or back-plate, which together form a second capacitive plate. A back-volume is provided below the membrane. A first cavity located directly below the membrane. Interposed between the first and second electrodes is a second cavity. A plurality of bleed holes connected the first cavity and the second cavity. Acoustic holes are arranged in the back-plate so as to allow free movement of air molecules, such that the sound waves can enter the second cavity. The first and second cavities in association with the back-volume allow the membrane to move in response to the sound waves entering via the acoustic holes in the back-plate.